The present invention relates to a method for operating a combustion engine following a cold start. The combustion engine has a supercharger device, at least two combustion chambers, a fuel-injection device, which injects directly into each combustion chamber at least once per working cycle, and a gas-exchange valve control device, which controls gas-exchange valves in a variable manner. After the cold start, a rich fuel-air mixture is generated in the respective combustion chamber using the fuel quantity that is injected per working cycle. The combustion-chamber charges produced in this manner are ignited at a later point in the respective working cycle than charges of the same combustion chambers in a combustion engine that has reached its operating temperature, all other conditions being equal. After the cold start, the combustion engine is also operated with first valve overlaps of intake-valve openings and discharge-valve openings of one of the combustion chambers in each case that are greater than second valve overlaps of intake-valve openings and discharge-valve openings of the respective combustion chamber adjusted in a combustion engine that has reached its operating temperature. Furthermore, the present invention relates to a control unit according to the definition of the species in Claim 9. Such a method and control unit are described in German Patent No. DE 10 2007 056 216 B4.
The pollutant emissions of gasoline engines are greatly reduced with the aid of a controlled three-way catalytic converter. However, since this catalytic converter starts converting hydrocarbons, carbon monoxide and nitrogen oxides only at a specific operating temperature (light-off temperature), it is important to reach this temperature as quickly as possible after a cold start.
Higher demands are imposed especially by ever more stringent emission regulations mandated by law. For example, in the European approval-driving cycle roughly 90% of the entire cycle emissions are produced within the first 80 seconds.
In an effort to comply with the limit values, an attempt is usually made to heat the catalytic converter as rapidly as possible following a cold start of the combustion engine, for which a variety of methods are employed. The combustion engine is selectively operated at a poor mechanical efficiency for this purpose, so that the greatest possible energy component of the injected fuel is able to be converted into exhaust heat and used for heating the exhaust-gas aftertreatment system.
For support, some engines use a secondary air-injection in their operation, in which more fuel is injected into the combustion chamber than would be required for the stoichiometric conversion. Using an airflow (referred to as secondary air), which is injected into the exhaust manifold with the aid of a special pump, the unburnt fuel is then directly converted into heat in a chemical secondary reaction.
Downsizing concepts are employed to an increasing extent within the framework of the ever more stringent emission and consumption standards. Here, induction engines are replaced by engines featuring a smaller displacement (and possibly a lower number of combustion chambers), supercharging (mostly turbocharging), and a direct gasoline injection. Moreover, in the case of supercharged engines, what is commonly known as scavenging constitutes the state of the art, in which a higher fresh gas charge is able to be achieved by flushing the residual gas from the combustion chambers and increasing the mass flow to the turbine, in particular at low rotational speeds, thereby obtaining a spontaneous engine-power characteristic.
In engines that have combustion chambers with a shared exhaust manifold and ignition angles that are close together (less than or equal to 180°), a temporal overlap of the exhaust-valve opening may occur (especially in the case of 4, 5, 8 and 10-cylinder engines). In order to increase the charges of the combustion chambers, especially at low rotational speeds, a switch to a shortened valve-lifting curve is possible with the aid of an exhaust-valve lift control. This avoids a pushback of burnt residual gas by a combustion chamber that discharges in parallel.
A method for heating a component of an exhaust-gas aftertreatment device of a combustion engine is described in German Patent No. DE 10 2009 012 336 B3. This combustion engine has a gas-exchange valve control by which a closing of at least one exhaust valve of a combustion chamber is alternatively able to be set to an advanced or retarded closing instant. During a normal operation of the combustion engine and during the overlap phase of the gas-exchange valves of a first combustion chamber, the exhaust valve of a second combustion chamber that discharges into the same exhaust manifold as the first combustion chamber and that follows the first combustion chamber in the ignition sequence, is opened. The opening generates a pressure surge in the exhaust manifold. This pressure surge counteracts a flow of air from the first combustion chamber into the exhaust manifold, which is undesirable during a normal operation since it increases the pressure in the exhaust manifold. For an operation that takes place after a cold start, and thus before a normal operation with a combustion engine that has reached the operating temperature, this related art does not provide for the described opening of the second exhaust valve, which takes place during the valve overlap in the first combustion chamber. As a result, the described pressure surge is omitted, and an entry of air or of air and fuel into the exhaust manifold takes place. Through exothermal reactions of the fuel with the air, a component of the exhaust-gas system, e.g., a catalytic converter or a particle filter, is meant to be heated.
There is still a need for operating strategies aimed at more rapid heating of the exhaust-gas system of a supercharged gasoline engine.